1. Field of the Invention
The present invention relates to a color separating unit having an improved color separation structure and a projection type image display apparatus employing the same which can illuminate a color light beam at a high optical efficiency and make a compact optical system.
2. Description of the Related Art
In general, a projection type image display apparatus is a system for providing a large picture by magnifying and projecting an image, which is generated from a micro display, that is, a light valve, such as a liquid crystal display or a digital micromirror display, onto a screen by using a projection optical system.
The image display apparatus can be classified into a one-panel type and a three-panel type according to the number of the employed micro displays. The three-panel type image display apparatus which uses three light valves arranged on optical paths separated for red, blue, and green has a high optical efficiency, but optical configuration thereof is complex and a manufacturing cost is high, compared to the one-panel type image display apparatus.
The one-panel type image display apparatus typically adopts a color wheel to periodically change an input white light beam into red, blue, and green beams, which is advantageous in the simplified configuration. However, by adopting the color wheel, loss in the amount of light by ⅔ occurs so that the optical efficiency is lowered compared to the three panel type. To address the above problem, an improved one-panel type image display apparatus has been suggested as shown in FIG. 1.
Referring to FIG. 1, in a conventional one-panel type image display apparatus, an unpolarized white light beam is generated and radiated by a light source 11. The radiated white light beam becomes a uniform beam while passing through a fly eye lens array 13 for mixing an input beam to make a uniform beam and then proceeds toward a polarization conversion system 15. The polarization conversion system 15 converts the polarization direction so that the unpolarized white light beam emitted from the light source 11 becomes a white light beam having one polarization direction. The white light beam passing through the polarization conversion system 15 is separated into red, blue, and green beams by first and second dichroic mirrors 17 and 19. That is, the first dichroic mirror 17 reflects a blue wavelength beam of the input white light beam and transmits the other beams thereof. The transmitting beam is separated into a green beam and a red beam by the second dichroic mirror 19.
First through third scanning prisms 21, 23, and 25 which periodically scrolls the input beam are arranged on the optical paths for the respective separated colors. Each of the first through third scanning prisms 21, 23, and 25 has a rectangular column shape and is rotated by a driving source (not shown). As each of the first through third scanning prisms 21, 23, and 25 is rotated by the driving source, an angle formed by an optical axis and a side wall of the prism changes on the optical path so that the proceeding path of the light beam passing through the prism changes periodically.
The initial angles of the first through third scanning prisms 21, 23, and 25 are set such that the light beams transmitting the first through third scanning prisms 21, 23, and 25 are radiated by dividing an effective image area of a display device 33 into three regions while the first through third scanning prisms 21, 23, and 25 rotate on the optical path. Thus, according to the driving state of the first through third scanning prisms 21, 23, and 25, as shown in FIG. 2, the separated color light beams are scanned in the effective image area of the light valve 33 by repeating an order of (B, R, G)→(G, B, R)→(R, G, B).
The light beams passing through the first through third scanning prisms 21, 23, and 25 are synthesized by the third and fourth dichroic mirrors 27 and 29. Reflection mirrors 18 and 20 are arranged between the first dichroic mirror 17 and the third dichroic mirror 27, and the second dichroic mirror 19 and the fourth dichroic mirror 29, respectively, to change the proceeding path of the light beam.
The light beam scrolled and passing through the fourth dichroic mirror 29 is incident on a polarizing beam splitter 31 which transmits or reflects the incident beam according to the polarization of the light beam. The light beam reflected by the polarizing beam splitter 31 is periodically scrolled, as shown in FIG. 2, and incident on the light valve 33. The light valve 33 controls the incident beam in unit of pixels to form an image. The image is generated by changing the polarization of an output beam in unit of pixels. The light beam having a polarization different from that of the incident beam transmits the polarizing beam splitter 31 and proceeds toward the projection lens unit 35. The projection lens unit 35 magnifies and projects the input image onto a screen 50.
The image display apparatus includes a plurality of relay lenses 41, . . . , 48 on the optical path to transfer the light beam emitted from the light source 11 to the light valve 33.
However, although the optical configuration of the conventional image display apparatus adopts the single light valve to realize a color image, the optical configuration thereof is very complex.